Chemical analysis of water, foodstuffs like milk and biological fluids such as serum and urine is often desirable or necessary. Various analytical elements to facilitate such analyses are known. Generally, such elements include a reagent (hereinafter termed interactive composition), for a substance under analysis (hereinafter termed analyte). The interactive composition, upon contact with a liquid sample containing the analyte, effects a detectable change in response to the presence of the analyte. For example, such a detectable change can be the formation or disappearance (e.g. reduction) of a detectable species, e.g. a dye. Such a change can be determined as it occurs (i.e. a rate assay), or after a certain time (i.e. endpoint assay).
Recently, much work has been directed toward developing dry analytical elements useful in diagnostic chemical analysis of biological fluids which provide highly quantitative results quickly and conveniently. For example, U.S. Pat. No. 3,992,158 (issued Nov. 16, 1976 to Przybylowicz et al) describes integral analytical elements which are a significant advance in the clinical chemistry art. These elements generally contain an isotropically porous spreading zone, also known as a fluid metering zone, which uniformly distributes a liquid sample throughout the zone. A variety of useful spreading zones are described in that reference. In one embodiment, the spreading zone is composed of a "blush" polymer which has a particulate material dispersed therein. Such elements are manufactured using conventional coating and drying techniques, and subsequently slitting or chopping the dried layer. However, during such mechanical handling operations, it has been observed that the dry blush polymer layers are sensitive to abrasion, and objectionable flaking or dust is generated particularly during slitting and chopping operations. Not only is the dust an environmental hazard, but this abrasion sensitivity often leads to unacceptable product when the dried spreading layer has ragged edges or crumbles during slitting. Attempts have been made to improve spreading layer abrasion resistance by increasing the blush polymer coverage or by reducing the amount of non-solvent (defined hereinbelow) used during the coating operations. These efforts, however, have had limited success, and undesirably resulted in a loss of zone porosity or void volume and attendant long liquid spreading times.
U.S. Pat. No. 3,014,810 (issued Dec. 26, 1961 to Dybalski et al) relates to pigment compositions which can be incorporated in nonporous rubbers and plastics to provide nonporous finished products having improved hardness, tensile strength and other physical properties. The pigment compositions described therein include a mixture of two amine surface-active agents in a specified ratio. These surfactants allegedly reduce pigment agglomeration during storage and shipping and improve pigment dispersion in the hard rubbers and plastics. The first agent is a quaternary ammonium compound and the second is a tertiary amine. The mixture of surface-active agents described therein is not suitable for use in analytical elements containing various chemical reagents because the tertiary amine of the mixture is highly reactive with respect to reagents commonly employed in clinical chemistry analyses. There is no suggestion in this reference of any means for improving the cohesive strength of porous materials such as porous spreading layers used to transport liquids.
Hence, there is a need in the art for a porous spreading zone material which has improved cohesive strength and abrasion resistance but which retains desirable spreading properties and is inert to chemical reagents which may be incorporated therein.